Ventilation vs Energy Efficiency

Ventilation does not mean uncontrolled draughts!

Ventilation is key for all buildings, after all we do need a bit of the fresh air stuff to stay alive! However for older buildings it is even more important. Solid walled buildings should be regarded as having ‘moist’ walls and hence they dry to the inside as well as the out (assuming that they have not been covered with cement render!) They therefore allow moisture into the internal environment and this needs to be wicked away along with all the moisture that we bring into our homes.

To illustrate this one study found that in a house we produce the following daily:

1.25 litres per person (just by breathing and sweating)

2.40 litres by showering, bathing etc

1.75 litres from a gas oven

1.00 litres from plants (assuming 25-30 plants per house)


So we are looking at around 10 litres of water per day from a family of four. All this water needs to be removed from the house and this requires ventilation.

Understanding how this works is really important, but actually very difficult. Where does the fresh air come from? Draughts around doors and windows? Chimneys? Punctures through the walls that have been poorly sealed? Where does it leave the house? Mechanical extracts? Chimneys? Trickle vents? Windows? Lofts?

Housing Associations and others are now wising up to the need for ventilation (after all many of their complaints are about damp and mould). Most are now fitting 2 x low energy vents in houses that run constantly. These only cost around £5 a year to run and can help tremendously with ventilation, but still people will turn them off! This is because people associate ventilation with increased energy consumption and hence costs.

Energy efficiency though is affected by poor ventilation. If walls are wet due to poor ventilation then the walls will be less efficient (by a factor of around 30%). So actually having good ventilation can help with energy efficiency. However most importantly ventilation is important for the health of the occupants and the health of the building itself. Without ventilation we are more likely to end up with mould and damp issues and this then leads to additional expense dealing with this. So it is a false economy in not venting well.

Ventilation is therefore linked to energy efficiency in several ways

So how to go about this?

Well there is no easy answer as each house is different (construction, levels of moisture, knowledge of occupants, lifestyles etc etc). So the best way is to use tools that measure what is actually happening and then automatically adjust ventilation rates.

Modern systems monitor and control a range of factors like temperature, humidity and CO2

These systems are not cheap and generally require a whole house solution with heat recovery (MVHR). These systems require vents to be placed in each room for either extract or input (or both). These are then combined (via ventilation ducts) in a central unit where the outgoing warm moist air help to pre-warm the incoming fresh air. These systems can work really well, but they need the house to be well sealed before they are fitted. They also require the ducts to be passed through the house etc. So these are quite complicated systems, but certainly the most effective as they help save the warm air in the house and therefore are more energy efficient.

Older houses of course are more difficult as they are harder to make airtight. However due to improvements over time many of these houses are now lacking in ventilation. People have fitted doubled glazed windows and doors (many of which have not had trickle vents specified), chimneys have been blocked up, suspended floors have been replaced with solid, lofts have been insulated, hatches draught proofed etc.

This has meant that whilst many houses do not have the required airtightness for efficient MVHR they are lacking in the required background ventilation to remove the 10 litres of water a day needed. Rock and a hard place territory!

So the exercise becomes a little vague and distinctly difficult, especially where people are not used to having (or paying for) pressure tests on their homes. So we are in the territory of ‘guestimating’. This is not ideal, but it is really poorly understood area of housing. So you may well find that you are the best person to make decisions on this, after all you will know how bad issues like condensation, mould and stale air are in the house and whereabouts it occurs most and when. So spend a little time thinking about where any problems are and what the root cause may be.

Windows and trickle vents

Chimneys and vents

Draught proofing measures

Extract fans and their direction of venting

Floor ventilation

Airflow in and between rooms (is furniture hard against walls? Is there an airgap at the top or bottom of internal doors? Are certain areas sealed off from the rest of the building?)

Functions of rooms (bedrooms, kitchens and bathrooms are subject to high humidity)

Thermal bridging / cold spots (these attract condensation)

Uncontrolled ventilation via major draughts around doors and windows, floors etc is not the answer though. This can be a real drain on resources as you cannot adjust the airflow to meet the ventilation needs of the building, so make sure that any major draughts are dealt with properly, but be aware that by reducing the uncontrolled airflow that you should be thinking about installing controllable airflow. For example, if you are replacing some old draughty windows then replace them with well sealed units that have adjustable trickle vents on.

Many social housing providers now also fit Positive Input Ventilation systems to reduce humidity levels in their houses. These systems can be simple and easy to install in houses with lofts. Basically they pump air from the loft into the house constantly and the warm moist air is pushed out of the various holes in the housing envelope. They have had a variety of reviews, but those houses that suffer most from high humidity / condensation issues seem to see the best results. I may well try a system in my house and then I can speak with a bit more authority!

Personally I think that buildings need to have:

– extracts in the main moisture generating areas, i.e. kitchen and bathrooms (these should be through walls that are not subjected to the prevailing winds and where this is necessary they should have vent covers installed).

– background trickle vents in windows

– where no trickle vents then 2 x background vents should be installed, ideally these will be heat recovery units that can exchange warmth from the stale air and impart it to the incoming fresh air

– all chimneys should be vented (both at the top and at each breast)

– ideally the whole wall structure should still be breathable

– suspended floors should be vented correctly

– all loft space should be vented correctly

– potentially a Positive Ventilation System installed

This should be treated as guidance to minimise the risks associated with poor ventilation. Most importantly remember that ventilation is not an ‘enemy’ of energy efficiency. It is a requirement of a healthy house.